The present invetion relates to a pipe coupler for a hose or pipe containing fluids. A typical example of the pipe coupler is shown in FIG. 4, in which the valve member 41 provided in a socket 40 is opened and closed by engagement and disengagement with a plug. The valve member 41 includes outer and inner surface grooves 42 and 43, into which O-rings 44 and 45 are fitted to maintain a sealing engagement with the outer periphery of the plug or with the inner face of the socket. However, the grooves for the O-rings are provided by a time-consuming and troublesome cutting process. In addition, difficulties are encountered not only in maintaining an entirely sealed condition at all times, but also in cutting into the coupler of such a structure. For these reasons prior art couplers could not be constructed to be small in size. Moreover, in the embodiment as illustrated in FIG. 4, the O-rings are constantly transformed under compression loads, and are lacking in the recovery of elasticity. A seal provided by the O-rings is also limited to the radial inside or outside dimension of the valve member 41. To seal both the inner and outer surfaces of the member 41, the O-ring should be fitted into the grooves formed in both the inner and outer walls thereof. Thus, the valve member should be long enough to provide space for such grooves. With this arrangement, sealing between the outer surface of the valve and the inner surface of the socket was incomplete, when the valve was opened.